They are a mystery for some reason. For wtcc 87 bmw the game doesn't even list peak torque (whaddaya mean 'N/A', chief?).

Yes, they are useful for setups and shift timing. The best you can do is to figure out what engine is in the car and google for real life torque curves, or compare motec logs between different setups (not trivial).

Mod makers should be able to state what their engines do. They may perhaps even know how to read the curves of built-in cars.

You must have found them at rscnet.org (which is down) under Engine Graphs for GTR Evo - RSC Forums. Thanks for saving them.
The notes.txt makes it clear that he had access to some kind of tables and interpolated the rest.

It's glad to see that I am not the only one who thinks that the shift lights are total hack jobs: